Initial Values

The default value for a property, when it’s not specified explicitly or
inherited, is called the initial value. The initial
value of each property is defined in the CSS specification.

Browsers have user agent style sheets that define the default rendering
of the various HTML element types. In some cases, the rules in those
built-in style sheets define values other than the initial values from the
CSS specification. For example, links are usually underlined, even though
the initial value for the text-decoration property is
none.